This invention relates to reactor vessel closure head assemblies and, in particular, to an integrated head assembly for a pressurized light water reactor.
In a typical pressurized water reactor (PWR) power plant, various mechanical components and systems are installed on the reactor vessel closure head. These mechanical components and systems include, for example, a control rod drive mechanism (CRDM) cooling system, a reactor vessel closure head lift rig, CRDM seismic restraints, and a CRDM missile shield. Each of these components is typically designed and installed as a permanent fixture to perform designated functions during plant operation. However, during refueling of the reactor these components have to be disassembled in order to remove the reactor vessel closure head from the reactor vessel. These components are stored in designated storage areas, generally inside the reactor containment. Typically, in a PWR plant, a series of steps are followed before the reactor vessel closure head is removed from the reactor vessel. The operational steps that are performed prior to detensioning the reactor vessel closure head studs include some or all of the following:
Remove and store heavy concrete missile shields.
Remove and store the CRDM cooling ducts.
Remove the seismic restraints.
Disconnect and store the CRDM power and rod position indicator cables.
Install the reactor head lifting rig tripod.
Remove cable trays and cables running from the reactor head to the operating deck or walls.
Disconnect heated junction thermocouples, nuclear steam supply system instrumentation, monitoring system cables, and reactor head vent lines.
Install temporary lead shield blankets around the vessel closure head area.
The procedure also requires that the nuts and washers be removed from the reactor vessel closure head and placed in storage racks during preparation for refueling. The storage racks are then removed from the refueling cavity and stored at convenient locations inside containment prior to reactor vessel closure head removal and refueling cavity flooding. The above steps are then reversed while reinstalling the reactor vessel closure head and the related reactor systems.
Each of these steps contributes significantly to the total cost associated with refueling the reactor. The total costs include costs associated with personnel man-hours required to perform the refueling, power plant down time and consequent loss of electricity production, radiation exposure to personnel, and potential human errors. In addition, the various components that must be removed for refueling activities require a large amount of the limited storage space available inside containment and raise the risk of inadvertent contamination of work and storage areas.
Concepts and designs for integrating some of the reactor vessel closure head systems into a modular integrated head design have been proposed. For example, in U.S. Pat. No. 4,678,623 to Malandra et al., a modular head assembly is disclosed wherein vertical lift rods are attached to the reactor vessel lifting lugs, and a missile shield, seismic support platform, CRDM cooling system, and lift rig are supported by the lift rods above the reactor vessel closure head. Because most or all of the modular head assembly taught by Malandra et al. is supported by the lift rods, however, very large loads are concentrated at the clevis connection at the reactor vessel closure head lifting lugs, which may cause damage to the lifting lugs and/or the body of the reactor vessel closure head. In addition, very heavy components such as the missile shield and the fans are supported at the distal ends of three relatively long lift rods, resulting in an unstable structure that may subject the lift rods to undesirable compressive, bending and torsional stresses. Malandra et al. also does not provide a structure for putting a shroud around the CRDMs.
In U.S. Pat. No. 4,830,814, Altman discloses an integrated head package having a missile shield that is slidably mounted near the distal end of three lift rods connecting to the reactor vessel closure head lifting lugs. A shroud is shown disposed about the CRDMs. Similar to the apparatus disclosed by Malandra et al., however, the heavy missile shield and lifting rig are installed at the distal end of three elongate lift rods that are connected at their proximal end to the reactor vessel closure head lifting lugs. The Altman apparatus, therefore, will also produce relatively high local loads in the reactor vessel lifting lugs and head. Altman also does not disclose any system for cooling the CRDMs.
There is a need, therefore, for an integrated head assembly for a pressurized water reactor that can be removed from the reactor vessel integrally with the reactor vessel closure head, and that does not introduce undue local stresses at the reactor vessel closure head and lifting lugs.
The present invention is directed to an apparatus and method that satisfies this need. The apparatus includes an integrated head assembly for a pressurized light water nuclear reactor having a lift assembly that engages the lifting lugs on the reactor closure head. A support structure is provided above the reactor closure head with a shroud assembly and a baffle structure attached thereto. At least one upwardly extending duct for a CRDM cooling system is also provided. The apparatus includes a seismic support system and a missile shield attached to the support structure and disposed generally above the control rod drive mechanisms. At least one cooling air fan is fluidly connected to the duct.
In an embodiment of the invention the duct is cooperatively formed by the baffle and the shroud assemblies.
In an embodiment of the invention the support structure includes a ring beam with a number of saddle members that sits atop the reactor vessel closure head. The ring beam may be formed from three annular segments that are joined end to end. The support structure may also include a cylindrical support grid that extends upwardly from the ring beam. The shroud assembly may also comprise multiple axial segments, and provide air inlet port(s) for the air cooling system. In the preferred embodiment the air cooling system includes an upper plenum interconnecting three cooling fans and two vertical ducts.
An embodiment of a method for retrofitting a pressurized water nuclear reactor according to the present invention includes shutting down the nuclear reactor and removing the reactor vessel closure head from the reactor vessel and placing it on a reactor head stand. Lift rods are then attached to the lifting lugs on the reactor vessel closure head. An integrated head assembly module is then installed, the module including a ring beam that rests atop the reactor vessel closure head, a shroud assembly that sits atop the ring beam, and a baffle assembly attached to the shroud assembly. A seismic support system is then connected to the control rod drive mechanisms and a missile shield is installed above the CRDMs. A lifting assembly is then operatively attached to the lift rods above the missile shield, and the reactor vessel closure head is reinstalled on the reactor vessel.